1. Field of the Invention
The present invention relates to a method and apparatus for die casting molten material, and more particularly, to a method and apparatus for injecting a shot of molten material into the cavity of a die.
2. Description of the Related Art
Die casting is frequently used as a method for forming articles from molten material. The present invention will be described in terms of casting molten metal; however, it should be understood the invention may be practiced with other materials which may be cast from an initially liquid state. Typically, two or more die parts are provided such that, when brought together, they form a cavity which defines the shape of the article to be cast. Molten metal is introduced into the cavity and allowed to cool. If desired, the metal may be squeeze cast under high pressure to yield a heat treatable or weldable casting. The die parts are opened and the cast article is removed.
In the past, molten metal has been introduced into a die by means of a shot sleeve. FIG. 1 shows a horizontal die casting apparatus with a shot sleeve arrangement according to the prior art. The die 10 includes an ejector die 12 mounted to a movable platen 14 and a cover die 16 mounted to a stationary platen 18. Together, the dies 12 and 16 form a cavity 19 into which a shot of molten metal will be introduced. A cylindrical shot sleeve 20 is disposed passing axially through the stationary platen 18 and the cover die 16 in fluid communication with the cavity 19. The upper surface of the outer wall near the end of the shot sleeve 20 is penetrated by an open pouring or filling hole 22. Molten metal 24 is ladled through the filling hole 22 into the interior of the shot sleeve 20.
A plunger 26 seals off the outer end of the shot sleeve and reciprocates within the shot sleeve 20 to inject the molten metal into the die. The plunger 26 is connected axially to a plunger rod 28, crosshead adapter 30, and shot cylinder 32. The shot cylinder 32 is typically a hydraulic cylinder having a reciprocating shot cylinder rod 34 which causes the plunger 26 to advance toward the die 10 and withdraw therefrom. The outer end 36 of the shot cylinder rod is threaded to allow for adjustment of the shot size and stroke length.
Die casting methods and apparatus according to the prior art are subject to problems arising from the open filling hole 22 of the shot sleeve 20. The molten metal within the shot sleeve 20 is free to exit through the filling hole 22 until the plunger 26 advances past the filling hole. If the shot sleeve were entirely filled with molten metal, the beginning of plunger stroke would cause molten metal to spurt out of the filling hole 22. Therefore, as shown in FIG. 1, the shot sleeve 20 can only be partially filled with molten metal prior to the injection stroke of the plunger 26.
Since the shot sleeve 20 can only be partially filled with molten metal, the diameter of the sleeve must be enlarged to provide an air space as well as for the necessary volume of molten metal. This enlargement of the shot sleeve diameter reduces the mechanical advantage of the shot cylinder 32, making the apparatus less suitable for squeeze casting.
FIG. 2 shows the effect of the injection stroke of the plunger 26 on the molten metal 24. Since the molten metal does not completely fill the interior of the shot sleeve 20, a rolling, turbulent wave 40 of molten metal is created. Such turbulence in turn causes the formation of air bubbles 42 within the molten metal. The air bubbles ultimately cause unwanted porosity in the castings.
Accordingly, there is a heretofore unmet need for a die casting method and apparatus that prevents molten metal from exiting the filling hole of the shot sleeve, eliminates turbulence and air entrainment in the metal, enables increased injection pressures by improving the mechanical advantage of the plunger, and that is readily adaptable to existing die casting equipment.